1. Field of the Invention
This invention relates to the production of terephthalic acid. More particularly, it relates to an enhanced process for the production of said terephthalic acid.
2. Description of the Prior Art
In a typical air or enriched air based process for producing terephthalic acid, liquid p-xylene is fed to a stirred tank reactor, with a monobasic aliphatic acid, typically acetic acid being used as a solvent. The ratio of solvent to reactant is typically one to ten weights of solvent per volume of reactant (1:1 to 10:1). The reaction is catalyzed with a heavy metal or mixture of heavy metals, most commonly cobalt and manganese in the form of acetate salts. In addition, bromine, in the form of bromic acid, is commonly used as an initiator. The reactor is maintained at an operating temperature of between 170.degree. C. and 225.degree. C. The operating pressure is generally between 100 and 300 psig. Compressed air or enriched air, typically having between 21% and 28% oxygen, is sparged into the bottom of the reactor. Oxygen from the air is dissolved into the liquid phase and reacts with the p-xylene to produce the desired terephthalic product. Intermediate oxidation products and by-products are also formed in quantities that depend on the reaction conditions employed. At a residence time of one hour, the conversion of p-xylene is typically about 99%, with the yield to desired terephthatic product being greater than 96%.
The most important intermediate oxidation product in the production of terephthalic acid (TPA) is 4-carboxybenzaldehyde (4CBA), which is one oxidation step removed from terephthalic acid. The presence of 4CBA in the TPA product is undesirable. It acts as a chain terminator in subsequent polymerization reactions which convert TPA to its most important end products, i.e., polyester fibers and polyethylene terephthalate resins. For a given residence time, the conversion of 4CBA to TPA has been observed to increase with temperature. Hence, the concentration of 4CBA in the TPA product decreases with increased operating temperature, so that TPA product quality increases at higher operating temperatures.
Raw material losses to undesirable by-products, on the other hand, also increase with temperature. The acidic acid solvent and, to a lesser extent, p-xylene, react to produce carbon dioxide, carbon monoxide, methyl bromide and methyl acetate, all of which are environmentally sensitive materials. Since a high reaction temperature must be maintained to make product terephthalic acid that meets applicable quality standards, the loss of acetic acid and the commensurate production of byproduct gases is usually a significant factor in the economics of the overall operation.
In such known operations, feed air must be compressed to a pressure somewhat above the reactor operating pressure before it is blown into the reactor through a pipe or other submerged sparger. As the air bubbles are dispersed in the reactor and are circulated through the body of liquid reactant and solvent by an agitator device, the oxygen concentration in the air bubbles decreases as the oxygen dissolves and reacts with the TPA. The residual air bubbles disengage from the liquid phase and collect in a gas space at the top of the reactor to form a continuous gas phase. This waste gas must be vented in order to provide space for fresh air feed, while maintaining adequate gas hold-up in the reactor to promote the desired oxygen transfer from the air to the liquid phase.
To avoid the possibility of fire or explosion, the oxygen concentration in the gas space at the top of the reactor must be maintained below the flammable limit. For practical operating purposes, the oxygen concentration must be maintained at less than 8-9% by volume. More typically, the oxygen concentration in the gas space is maintained below 5% by volume to provide a safe margin below the flammable limit. Thus, in a well stirred tank reactor, the average concentration of oxygen in the circulating air bubbles must be below 5% in order to insure that the average concentration of oxygen in the gas that collects in the headspace of the reactor is nonflammable.
The oxygen concentration in the gas space is a function of the rate at which air or enriched air is fed into the reactor and the rate of consumption of oxygen from the air by reaction with p-xylene. The rate of reaction and, therefore, the TPA production rate per unit of reactor volume, increases with temperature, pressure, oxygen concentration in the gas phase, p-xylene concentration, promoter concentration and catalyst concentration. Since the concentration of dissolved oxygen in the liquid phase and, hence, the reaction rate of oxygen, is proportional to the oxygen concentration in the gas phase, for a given set of reaction conditions, the 5% oxygen restriction in the headspace effectively limits the oxygen reaction rate.
As air bubbles circulate within the reactor, acetic acid, water, volatile organic chemicals (VOC's) and byproduct gases such as CO.sub.2, CO, methyl bromide and methyl acetate evaporate into the bubbles and collect in the continuous gas phase which is vented from the reactor. The total amount of volatile species which leave the reactor with the vent gas is proportional to total gas throughput, which is proportional to the air feed rate. The amount of byproduct gases which leave the reactor with the vent gas depends on their rate of formation.
The federal, state and local air quality standards which apply to a particular production facility determine the degree to which these species must be removed from the vent gas before it is released to the atmosphere. Acetic acid is a valuable solvent in the process so it is usually condensed and recycled to the reactor. Residual organic compounds are usually stripped from the vent gas which produces a liquid waste stream from the stripper bottoms. Some vent gas treatment systems may also include CO.sub.x and methyl bromide abatement systems to meet air quality standards. Since the total amount of material which must be removed from the vent gas is proportional to the air feed rate, the size of the vent gas treatment equipment and the amount of waste which is generated in the process is similarly proportional to the air feed rate.
Clearly, air or said enriched air, typically 21% to 28% oxygen, based TPA plant design requires optimization of temperature, pressure, catalyst loading, air feed rate, reactor volume, and vent gas treatment equipment. For example, increasing temperature increases productivity per unit reactor volume and improves product purity, but it also leads to yield and solvent losses, and byproduct gas formation due to over oxidation.
In the air based terephthalic acid production process as described above, a relatively high operating temperature is required in order to complete the oxidation of 4CBA to TPA, and thereby produce said TPA that meets applicable product quality standards. The high temperature required for product purity also results in significant reaction of acetic acid, and to a lesser extent of product p-xylene, to unwanted by-products, such as CO.sub.2, CO, methyl bromide and methyl acetate, is noted above. As those skilled in the art will appreciate, there is a significant operating cost penalty associated with providing makeup acetic acid to the process, and with loss of p-xylene reactant and the related disposal of waste material. There is also a significant environmental impact associated with the formation of CO.sub.x, methyl bromide, methyl acetate and other emissions.
In addition, in the air based process, there is a substantial capital and operating cost penalty associated with the compression of the nitrogen in the feed air stream. The nitrogen is inert and does not contribute to the efficiency of the reaction process. In addition, there is a significant capital and operating cost penalty associated with treating the vent gas. These costs are proportioned to the amount of nitrogen that is introduced into the reactor vessel in the air feed thereto.
It is an object of the invention, therefore, to provide an improved process for the production of terephthalic acid.
It is another object of the invention to provide a terephthalic acid production process reducing the amount of byproduct and waste gas generation.
With those and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.